Nitrobenzene is an important intermediate in the chemical industry which is required particularly for preparation of aniline and hence also for preparation of the di- and polyisocyanates of the diphenylmethane series and the polyurethanes based thereon.
The nitration of benzene with nitric acid to give a crude nitrobenzene has already been the subject of numerous publications and patent applications. The processes that are standard nowadays correspond essentially to the concept of adiabatic nitration of benzene by a mixture of sulfuric acid and nitric acid, which is generally referred to as mixed acid. Such a process was claimed for the first time in U.S. Pat. No. 2,256,999 and is described in current embodiments, for example, in EP 0 436 443 B1, EP 0 771 783 B1 and U.S. Pat. No. 6,562,247 B2. It is a particular feature of the processes with an adiabatic reaction regime that no technical measures are taken to supply or remove heat to or from the reaction mixture.
Isothermal processes for nitration of benzene with mixed acid are also known, as described, for example, in EP 0 156 199 B1.
Also known are processes, for example from U.S. Pat. No. 2,739,174 or U.S. Pat. No. 3,780,116, for the nitration of benzene that do not require the use of sulfuric acid.
In principle, gas phase processes for nitration of benzene with nitric acid or nitrogen oxides are also possible, but the yields achievable thereby at the present time are still low (EP 0 078 247 B1, EP 0 552 130 B1).
A common factor to all these processes is that the reaction product formed at first is a crude nitrobenzene typically containing, as impurities, nitric acid and—if nitration has been effected with mixed acid-sulfuric acid, water, benzene and, as organic impurities, dinitrobenzene, nitrated oxidation products of benzene, especially nitrated phenols (nitrophenols). The crude nitrobenzene may also contain organic compounds formed from the compounds that were present as impurities in the benzene used (WO 2008/148608 A1). In addition, the crude nitrobenzene often also contains metal salts that may be present in dissolved form in the acid residues or in the crude nitrobenzene (DE 10 2007 059 513 A1). These impurities are undesirable since they can adversely affect downstream processes in which nitrobenzene is used, for example the preparation of aniline. Suitable work processes which include wash and distillation stages are described, for example, in U.S. Pat. No. 6,288,289 B1, EP 1 593 654 A1, EP 1 816 117 B1 and WO 2011/021057 A1.
The quality of a reaction process for preparation of nitrobenzene is thus defined firstly by the content of unwanted secondary components and impurities in the crude product that arise from improper conduct of the reaction. Secondly, the quality of a reaction process is defined in that the entire process can be operated without technical production outage or problems that necessitate intervention in the process, and that losses of feedstocks are prevented or at least minimized.
Such problems can occur, for example, in the startup and shutdown of the nitration reaction. Problems of this kind may, for example, be that solids are formed that lead to caking and blockage in the equipment (nitration tank, heat exchangers, conduits, etc.). A further disadvantage is that, in the event that inspection, maintenance, repair and cleaning operations are necessary on or in a reactor or another plant section, it is regularly necessary always to switch off all plant sections since the process steps build on one another and hence always proceed successively. As a result, it is necessary to empty the entire plant, which leads to a considerable amount of reject material. Furthermore, energy has to be expended in order to bring reactors and plant sections back to the respective operating temperatures. Such production shutdowns for plant inspections, repair and cleaning measures or shortfalls of raw material or auxiliary that occur, whether planned or unplanned, are recurrent plant states which have a considerable influence on the economic operation of a plant or process that works continuously.
Although the prior art processes described succeed in preparing nitrobenzene with a high yield and without loss of quality in the end products, the only processes described are in the normal state of operation. Production shutdowns for plant inspections and repair and cleaning measures or, for example, shortage of raw material or auxiliary are not taken into account. At the same time, production shutdowns, planned or unplanned, are recurrent plant states which have a considerable influence on the economic operation of a continuously operating plant.
Such a production shutdown may be an inspection shutdown which is planned in advance, for which purpose the plant is run down, the energy supplies are switched off and typically all plant sections that are to be inspected are opened and cleaned for the purpose of examination. Such an inspection may take one or more weeks. After the inspection has ended, the production plant is closed, optionally inertized and provided with auxiliaries and, once the appropriate energy sources and raw materials are available, started up again. However, a production shutdown is not necessarily associated with opening or another mechanical intervention into a reactor or another apparatus in the plant, but may also be connected to the shutdown and restart of the production plant for various other reasons, for example in the event of outage of the raw material supply. In such a case, the plant is typically run in part-load operation and, in the worst case, when the logistical supply chain is interrupted, has to be shut down. Furthermore, production shutdowns may be forced by requirements for maintenance, cleaning or repair in the production plant. Shutdowns here in the nitrobenzene process are typically described as short when production is interrupted for up to one day. It is a feature of all these production shutdowns in practice that there are losses of production, and that, on restarting of the plant, for example when inertization is necessary, nitrogen is consumed or, in the heating of the plant or the feedstocks, forms of energy such as steam and power are required.
The person skilled in the art is aware that an industrial process operated semicontinuously or continuously proceeding from a production plant in operation cannot be switched instantaneously to a production shutdown, but has to be run down in a controlled manner beforehand. This is also the case for a plant outage in the event of an accident. In order to be able to produce again after the production shutdown, the plant has to be run back up to the process parameters before the production shutdown. Reactants and apparatuses have to be heated up, apparatuses may have to be inertized, and the loading of the apparatuses with the reactants is gradually increased to the desired target value. During this startup phase, there is thus still loss of production volume, and a disproportionate amount of energy has to be expended in order to prepare the cooled plant for startup and then to run it up to the desired target value with observation of all operationally relevant parameters as well.
What would thus be desirable would be a process for preparing nitrobenzene in which simple measures enable optimization of production shutdowns in the operation of the nitrobenzene preparation process in terms of time taken, energy consumption, auxiliary and raw material consumption and/or reduction in wastes. This would lead to a not inconsiderable degree of improvement in productivity or economic viability of a continuously operated process or a corresponding production plant.